This invention relates to an electronic imaging device that uses a solid-state imaging device, such as a CCD two-dimensional image sensor, to pick up images, and an electronic imaging device control method, and more particularly to an automatic focusing system capable of high-speed operation.
More and more electronic imaging devices, such as electronic still cameras, are being used these days. In the electronic imaging devices, the imaging optical system forms an image of the subject on a solid-state imaging device, such as a CCD two-dimensional image sensor, which converts the image into an electric signal to produce an image signal. The image signal is recorded on a recording medium, such as a semiconductor memory or a magnetic disk.
Such an electronic still camera is provided with an autofocus (AF) system which generally senses focal errors in the focus lens of the imaging optical system and adjusts the focal point automatically by moving the focus lens along the optical axis on the basis of information on the focal error. One known AF system for the electronic still camera is an imager AF system for sensing focal errors on the basis of the contrast of the subject picked up by an imaging device and judging the in-focus position.
More specifically, in the imager AF system, while the focus lens is being moved stepwise along the optical axis, the high-frequency components are extracted by a high-pass filter from the image signal obtained from the imaging device. The reason is that the contrast of the subject becomes the highest when the focal point has been adjusted correctly and simultaneously the high-frequency components of the image signals become the largest. Then, the amount of high frequency components corresponding to each position of the focus lens, such as accumulative add value of the high-frequency components, is compared with each other. The highest contrast point at which the amount of high frequency components peaks is determined to be the in-focus point. The focus lens group is then moved to the in-focus point. This AF system is called a so-called mountain-climbing system. Such a conventional AF system has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-168113 or Jpn. Pat. Appln. KOKAI Publication No. 9-200597.
The conventional AF system, however, has the following problem: one screen (one frame or one field) of image signals is needed for each focus lens position to sense focal errors at more than one focus lens position and judge the in-focus position and therefore the AF operation requires a long time.
For example, when the focus lens position is set at 24 steps, it is necessary for the imaging device to pick up 24 screens of image signals for AF and therefore the time required for the AF operation amounts to a 24 frame period or a 24 field period. Thus, it is impossible to follow a subject moving at high speed. Decreasing the number of steps at the focus lens position shortens the time required for an AF operation, but the AF accuracy decreases accordingly.
As described above, in the AF system in the conventional electronic still camera, one screen of image signals is needed for each focus lens position to sense focal errors and judge the in-focus position, which makes a high-speed AF operation impossible. In addition, when the number of steps at the focus lens position is decreased to speed up an AF operation, this causes another problem of decreasing the accuracy of the AF operation.